The present invention relates to motor control systems and, more particularly, to variable-speed motor control systems.
In my prior allowed U.S. patent application, Ser. No. 395,331, now U.S. Pat. No. 4,509,004, the disclosure of which is herein incorporated by reference, I disclose a motor control system for a variable speed motor employing a cycloconverter technique for gating fixed-frequency, 3-phase AC power to the windings of a variable speed induction motor in a sequence which produces a magnetic field in the motor rotating at a desired speed. A measurement of the shaft speed is added to a predetermined value of slip frequency to produce a driving signal. Power is gated to three motor windings through an array of 18 silicon-controlled rectifiers (SCRs), three on each winding for gating the plus portions of the three phases, and three on each winding for gating the minus portions of the three phases. The motor speed and torque are independently controlled. That is, the motor speed is controlled by combined speed and slip signals, whereas the torque is controlled by motor current. The control system varies the portion of each phase fed to the motor windings for controlling the motor current.
When SCRs are connected across AC lines, it is possible to accidentally trigger back-to-back SCRs to form a direct short between phases. As is well known, an SCR remains non-conducting until triggered into conduction by an appropriate signal at its gate electrode. Once in the conducting condition, it remains in the conducting condition until a voltage reversal at its anode and cathode terminals extinguishes such conduction for long enough to permit free carriers to be dissipated. Once conduction ceases, the SCR remains non-conducting until the trigger signal is again applied to its gate electrode. Even a very small residual current in an SCR is sufficient to permit full conduction to resume.
I employ a number of techniques in the referenced U.S. patent application to ensure that a previously conducting SCR is fully extinguished before permitting another SCR to begin conduction. For example, the plus voltages to all three of the motor windings flow through three windings of a first quad-winding transformer. Similarly, the minus voltages to all three of the motor windings flow through three windings of a second quad-winding transformer. When motor current flows through one of the windings in each of the quad-winding transformers, a powerful reverse voltage is induced in the other three windings to ensure that any residual current in the SCRs associated therewith is fully extinguished. In addition, at the time of transition from one motor winding to another, an appropriate extinguish pulse is applied to the fourth winding of the appropriate quad-winding transformer to provide further current extinguishment. A logic circuit generates the extinguish pulses for the plus and minus quad-winding transformers. The logic circuit further establishes the required relationships between the plus and minus extinguish pulses during forward and reverse motor rotation.
Another disclosed method for guaranteeing SCR extinguishment includes a sensitive motor current sensor whose output inhibits gating on a new SCR until the motor current provided by the previously conducting SCR is extinguished.
A further type of device for variable-speed motor drive from a fixed-frequency AC source employs a current source inverter (CSI) in which the AC source power is first rectified to produce DC power and the DC power is gated to the motor windings in a sequence effective for producing the rotating magnetic field.
Conventional CSI systems suffer from drawbacks which limit their usefulness. In particular, one type of CSI system is practical only with special motors having a low Q. That is, they are only useful with low-efficiency motors having a low ratio of winding inductance to winding resistance. In addition to the limitation imposed by the low-efficiency motor, such a conventional CSI system employs a complicated clamp circuit to avoid a chain of voltage increases in smoothing capacitors across the motor windings. The clamp circuit itself is an additional source of inefficiency.